\chapter{Requirements for advanced dune safety assessment}
\label{ch:assessment_requirements}

An advanced dune safety  assessment is performed when a detailed assessment is considered unsuitable for a certain location or stretch of dune coast. In general, the morphological behaviour at those locations is significantly influenced by physical processes that are lacking in the detailed assessment model (a 1-dimensional profile model), hence the need for a more in-depth advanced assessment. This is the case for approximately $40\%$ of the dutch coastline. The most common causes are listed below:

\begin{itemize}
\item \textbf{Flat profile slopes} Very flat beach and foreshore slopes are often gentler than the resulting profile of the detailed safety assessment, so no dune erosion is predicted. However, in practice the presence of long waves does cause dune erosion.
\item \textbf{Irregular profiles} Profiles containing pronounced irregular features like banks or gullies present a problem for the detailed safety assessment model, which imposes a prescribed shape on the final profile.
\item \textbf{Hybrid sea defenses} These sea defenses consist partly of sand and partly of a hard structure. (for instance sea walls, dune (foot) revetments, dikes with dunes in front).
\item \textbf{Hard objects} In particular at coastal cities there are often hard objects present on or in front of the sandy sea defense, while not functionally being part of the sea defense. These objects can however have an influence on the morphological behaviour of the sandy sea defense during storm conditions.
\item \textbf{Time varying storm surge} At locations with a relatively small first dune row, that might fail during design conditions, time varying storm conditions become essential for a proper assessment.
The moment of failure in view of the total storm influences the probability of inundation of the dune area or hinterland.
\end{itemize}

In the table below, the functional requirements to the WTI2017 version of XBeach are summarized and translated into physical processes the model should be able to resolve successfully.

\begin{table}[H]
  \caption{Translation of functional requirements into physical processes}
  \label{tab:functional_requirements}
  \begin{center}
    \begin{tabular}{ll}
      \hline
      Functional requirement & Physical process \\ 
			\hline
			Dune erosion & hydrodynamics, morphodynamics \\ 
			Gentle slopes & long waves \\ 
			Irregular profile shapes & process based model (no prescribed final profile) \\ 
			(banks \& gullies) & \\ 
			Structures \& objects & hard layers, scour holes\\ 
			Realistic storm forcing & time varying storm surge \& wave forcing \\ 
      \hline
    \end{tabular}
  \end{center}
\end{table}

The WTI2017 version of XBeach is supposed to be a solution to the above mentioned problems. To be able to show that XBeach is capable of doing so, the model results will be compared to large and small scale laboratory measurements and field data. Before doing this, the required functionalities have to be translated in to measurable physical aspects, on which the comparison will be made. This is shown in the table below, including the availability of measurements for comparison on each aspect.

\begin{table}[H]
  \caption{Overview of relevant physical processes in dune safety assessment}
  \label{tab:assessment_processes}
  \begin{center}
    \begin{tabular}{lccc}
      \hline
      Physical aspect & Experimental  & Experimental  & Field observations\\ 
                      & (small scale) & (large scale) &                   \\ 
      \hline
      \textbf{Hydrodynamics}	&  &  &  \\ 
      Water levels &  & \checkmark &  \\ 
      Wave heights &  & \checkmark &  \\ 
      Velocities   & \checkmark & \checkmark &  \\ 
      \hline
      \textbf{Morphology} &  &  &  \\ 
      Erosion volume & \checkmark & \checkmark & \checkmark \\ 
      Erosion profile & \checkmark & \checkmark & \checkmark \\ 
      Sediment concentration &  & \checkmark &  \\ 
			Hard layers/structures & \checkmark & \checkmark & \checkmark \\ 
			Scour holes & & \checkmark & \\ 
      \hline
    \end{tabular}
  \end{center}
\end{table}



